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LONDON — Scientists have succeeded in generating new stem cells in mice and say their success
opens up possibilities for the regeneration of damaged tissue in people with conditions ranging
from heart failure to spinal-cord injury.

The researchers used the same “recipe” of growth-boosting ingredients used for making stem cells
in a petri dish but introduced them instead into laboratory mice and found that they created
so-called reprogrammed induced pluripotent stem cells, or iPS cells.

“This opens up new possibilities in regenerative medicine,” said Manuel Serrano, who led the
study at the Spanish National Cancer Research Centre in Madrid.

Stem-cell experts not directly involved in the study said its success was exciting but noted
that the technique could not be used in humans because the reprogrammed cells lead to tumors in the
mice.

“Nobody wishes to do this for therapeutic purposes because this leads to the formation of tumors
called teratomas,” said Ilaria Bellantuono, a reader in “Stem Cell and Skeletal Ageing” at Britain’s
University of Sheffield.

But she added that Serrano’s work was “a proof of concept” that opened up the opportunity to
investigate ways to partially reprogram cells in the body up to a certain stage.

Stem cells are the body’s master cells and are able to differentiate into all other types of
cells. Scientists say that by helping to regenerate tissue, they could offer new ways of treating
diseases for which there are no treatments — including heart disease, Parkinson’s and stroke.

There are two main types of stem cells — embryonic stem cells, harvested from embryos, and adult
or iPS cells, cells taken from skin or blood and reprogrammed back into stem cells.

Serrano, who spoke to reporters ahead of his study’s publication in the journal
Nature on Wednesday, said one of his most-striking findings was that the iPS cells
generated in the mice seem to be more similar to embryonic stem cells than to iPS cells created in
petri dishes.

Specifically, he said, the iPS cells reprogrammed in mice showed the potential to differentiate
into more cell types than standard iPS cells or embryonic stem cells — suggesting that
reprogramming cells in a living mammal produces cells with greater potential.